Molding a plastic printing plaque

ABSTRACT

A flat, generally rectangular plastic printing plaque is made in a mold formed with a shallow, generally rectangular cavity for receiving and molding a plastic substance. The mold has a flexible portion and is movable between open and closed positions. A press encloses the mold and holds it closed under pressure to facilitate molding of the plastic substance. The relative conformation of the mold and press is such as to leave an insulating space between the flexible portion and the press. The mold is overcharged so that there is a total lateral deformation of the flexible portion which is within the range of 0.002 inches to 0.020 inches and preferably within the range of 0.006 inches to 0.010 inches.

United States Patent Sorensen et a].

[ MOLDING A PLASTIC PRINTING PLAQUE [72] Inventors: Robert L. Sorensen,Westport, Conn.; John Casanova, Racine, Wis.

[73] Assignee: Time, Incorporated, New York,

[22] Filed: Aug. 17, 1970 [21] Appl. No.: 64,244

[52] US. Cl. ..425/407, 264/313, 264/325, 425/410, 249/82 [51] Int. Cl..B29c 3/00 [58] Field of Search ..18/16 R, 16 E, 16 P, 16.5, l8/16.7,17H, DIG. 44, 5 P, 5 H, 5.3 H, 5.3 P, 12 F, 47 R; 249/82; 164/80; 264/325,313;

[451 Oct. 3, 1972 3,122,787 3/1964 Adams ..18/16 R X 3,278,654 10/1966Grandperret ..249/82X 3,389,198 6/1968 Tabe ..249/82X 57 ABSTRACT Aflat, generally rectangular plastic printing plaque is made in a moldformed with a shallow, generally rectangular cavity for receiving andmolding a plastic substance. The mold has a flexible portion and ismovable between open and closed positions. A press encloses the mold andholds it closed under pressure to facilitate molding of the plasticsubstance. The relative conformation of the mold and press is such as toleave an insulating space between the flexible portion and the press.The mold is overcharged so that there is a total lateral deformation ofthe flexible portion which is within the range of 0.002 inches to 0.020inches and preferably within the range of 0.006 inches to 0.010inches.

7 Claims, 13 Drawing Figures PATENTEDnma m2 3.695.807

SHEET 2 OF 4 MOLDING A PLASTIC PRINTING PLAQUE BACKGROUND OF THEINVENTION This invention relates to molding and, more particularly to anovel and highly-effective method and apparatus facilitating the moldingof a flat, generally rectangular plastic printing plaque.

Great advances in the molding of plastic printing plaques have been madein recent years. A patent to Sorensen et al. U.S. Pat. No. 3,505,14l,for example, discloses a molding of plaques of superior quality byovercharging a mold with a mold substance such as polyamide powder andmaintaining the mold substance therein under continuous pressure duringthe cooling thereof, so that the growth of shrink lines, bubbles andother imperfections, is inhibited during the cooling process.

SUMMARY OF THE INVENTION An object of the invention is to provide afurther advance in the molding of plastic printing plaques and, moreparticularly, to provide with even greater reliability for the moldingof printing plaques that are free of shrink marks, bubbles and otherimperfections.

The foregoing and other objects are attained in accordance with theinvention by the provision of molding apparatus for making a flat,generally rectangular plastic printing plaque. The molding apparatuscomprises mold means formed with a shallow, generally rectangular cavityfor receiving and molding a plastic substance. The mold means has aflexible portion and is movable between open and closed positions. Pressmeans is provided for enclosing the mold means, holding the mold meansin the closed position under pressure, and facilitating molding of theplastic substance in the mold cavity to form a printing plaque. Therelative conformation of the mold means and the press means is such asto leave an insulating space between the flexible portion and the pressmeans. The mold is overcharged so that there is a lateral deformation ofthe flexible portion which is within the range of 0.002 inches to 0.020inches and preferably within the range of 0.006 inches to 0.010 inches.

BRIEF DESCRIPTION OF THE DRAWINGS An understanding of additional aspectsof the invention can be gained from a consideration of the followingdetailed description of representative embodiments thereof, inconjunction with the appended figures of the drawing, wherein:

FIG. I is a schematic perspective view of a first embodiment of moldingapparatus constructed in accordance with the invention;

. FIG. 2 is a fragmentary sectional view in elevation on a larger scale,taken generally in the plane 2- -2 of FIG. 1 and looking in thedirection of the arrows and showing the apparatus at a first stage ofthe production cycle;

FIG. 3 is a fragmentary sectional view similar to FIG. 2 showing themold apparatus at a later stage of the production cycle in a firstmethod according to the invention;

FIG. 4 is a fragmentary sectional view similar to FIG. 3 showing theapparatus at a still later stage of the production cycle;

FIG. 5 is a fragmentary sectional view similar to FIG. 3 showing theoperation of the apparatus in a second method in accordance withtheinvention;

FIG. 6 is a fragmentary sectional view similar to FIG. 4 but showing astagein the production cycle of which an earlier stage is illustrated inFIG. 5;

FIGS. 7 and 8 are fragmentary sectional views similar to FIGS. 4 and 6showing other embodiments of apparatus in accordance with the invention;

FIGS. 9-12 are fragmentary sectional views in elevation showing defectsin the operation of certain conventional molding apparatus which areovercome in accordance with the present invention; and

FIG. 13 is a fragmentary section view in elevation showing a defectiveproduct produced by the conventional apparatus of FIGS. 9-12.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Molding apparatus according tothe invention is adapted to Itiake aflat, generally rectangular plasticprinting plaque. The apparatus comprises mold means 10 shownschematically and in perspective in FIG. I and 'in sectional elevationin FIGS. 2-8. The mold means 10 is formed with a shallow, generallyrectangular cavity 12 (FIG. 2). In all of the embodiments, the moldmeans 10 is movable between open and closed positions, the open positionbeing illustrated in FIG. 2 and the closed position being illustrated inFIGS. 1 and 3-8.

The mold means 10 in the open position is charged with a weighed amountof plastic granules commonly referred to as molding powder 14 (FIG. 2).The granules are discrete pellets or particles of material and the pilecontains voids filled with air. The powder volume exceeds the normalvolume of the cavity 12 to be formed by the mold top 15 and base 15'.

When the mold is shut and heated to melt the plastic 14, the moltenplastic fills the cavity 12, and in its melt condition the volume issufficient to bend downward the flexible wall 16 (FIG. 3). The maximumdeformation may or may not be limited by abutment of the flexible wall16 and the platen 20. In any case, the mold means 10 is overchargedsufficiently that the total lateral deformation d of the flexibleportion 16 is within the range of 0.002 inches to 0.020 inches andpreferably within the range of 0.006 inches to 0.010 inches.

As the plastic cools to a solid state (FIG. 4), its volume decreasesbecause of three factors:

a. changing from the liquid state to the solid state;

b. development of orientation or crystallinity within the solidstructure; and

c. contraction as the temperature of the mold substance decreases. Inconnection with factor (c), the thermal coefficient of expansion andcontraction for plastics often exceeds that of metals by a factor of 10or more. The mold assemblies are made of metal for reasons of permanenceand good heat transfer characteristics, so that the change of the moldvolume for a given temperature change is less than the change in volumeof the plastic for the same temperature change.

If the plastic volume is permitted to become less than the mold volumeduring solidification and normalizing,

cooling and at the same time accommodates the volume changes.

The mold means flexible portion may comprise a single flexible wall 16or, min the embodiment of FIG. 8, two flexible walls 16 and 16'.

Press means 18 is provided and includes a lower press platen 20 and anupper press platen 22, the latter being illustrated in FIG. 8. The pressmeans 18 encloses the mold means 10, holds the mold means in the closedposition under pressure, and facilitates molding of the plasticsubstance 14 in the mold cavity 12 to form a printing plaque.

In accordance with the invention, the relative conformation of the moldmeans and the press means 18 is such as to leave an insulating spacebetween the flexible wall 16 (FIGS. 2-7) or the flexible walls 16 and16' (FIG. 8) of the mold means 10 and the press means 18. The insulatingspace may comprise a single compartment 24, as illustrated in FIGS. 3-7,or two com partments 24 and 24, as illustrated in FIG. 8.

In the embodiments of FIGS. 2-7, the flexible wall 16 can deformlaterally into the compartment 24; in the embodiment of FIG. 8, theflexible walls 16 and 16 can deform laterally into the compartments 24and 24', respectively. Such deformation takes place, as indicated above,because the cavity 12 is intentionally overcharged, and closing of themold means produces a bulge at the center of the flexible wall or wallsto accommodate the excess charge. Upon contraction of the plastic as itcools, the final product is somewhat thicker at the center than at theedges (FIG. 4) or is of uniform thickness throughout (FIGS. 6-8),depending on the initial charge.

An important feature of the present invention is that, whether theinsulating space is represented by a single compartment adjacent to oneflexible wall of the mold or two compartments respectively adjacent toopposite flexible walls of the mold, the total lateral deformation ofthe flexible portion into the insulating space is within the range of0.002 inches to 0.020 inches and preferably within the range of 0.006inches to 0.010 inches. By a lateral deformation is meant a displacementof the flexible wall, or a part of it, in a direction perpendicular tothe plane in which the flexible wall normally lies. By a totaldeformation is meant the displacement of the one flexible wall, if thereis only one, or the sum of the displacements of the two flexible walls,if there are two.

The relationship set forth above is valid regardless, within widelimits, of the major dimensions of the rectangular plaque. A typicalsuch plaque may measure, for example, 9 X 12 inches and be on the orderof 0.082 inch thick.

For a very large or small plaque, however, some deviation from therelationship set forth above may be tolerated. In particular, goodresults are obtained so long as the total'lateral deformation of theflexible wall or walls into theinsulating space or spaces is at maximumsubstantially equal to 1/5 of 1 percent and preferably l/ 10 of 1percent of one of the major dimensions of the plaque or so long as thetotal lateral deformation of the flexible portion into the insulatingspace is at maximum substantially equal to 20 percent and preferably 10percent of the thickness of the plaque.

If the flexible portion of the mold means 10 comprises a single flexiblewall, the insulating space comprises a single compartment and the wallcan deform laterally into the insulating space within the entire rangeupto the maximum specified. On the other hand, if the mold means 10comprises two flexible walls, then the insulating space comprises twocompartments 24 and 24 of equal depth, each being equal to half thedepth that would otherwise be employed, so that the compartmentsrespectively permit deformation of each of the walls within half of thespecified range. That is, each of two flexible walls is capable of alateral deformation which at maximum is within the range of 0.003 inchesto 0.005 inches; or each of two flexible walls is capable of a lateraldeformation which at maximum is substantially equal to l/20 of 1 percentof one of the major dimensions of the plaque; or each of two flexiblewalls is capable of a lateral deformation which at maximum is equal to 5percent of the thickness of the plaque.

In practicing the method of the invention, the mold cavity 12 isintentionally overcharged, as indicated above. By changing the weight ofthe plastic charged into the mold cavity, one can change the finalconfiguration of the solid plastic plaque. For example, to make aplastic plaque measuring 9 X 12 X 0.082 inches, a thick center isproduced by charging the cavity 12 with 171 grams of light-sensitivepolyamide or polyurethane molding powder containing a volatile material(moisture) substantially equal to 1.5 percent of the weight of thecomposition. The resulting plaque is 0.003 inch thicker at the centerthan at the edges. To produce a plaque of uniform thickness, the chargeis reduced by 5 grams.

The entire molding operation may be performed employing only a singlepress means, which is cycled between heating and cooling phases, andthis is quite I satisfactory for small-scale production, as in alaboratory. However, for high-volume commercial applications, it is moreeconomical to have a hot press and a cold press between which the moldmeans 10 is transferred. This avoids the necessity of alternatelyheating and cooling large masses of metal and results in a saving oftime and power requirements and hence in a reduction in cost.

In this embodiment of the invention, the compartments 24 and 24 serve anadditional function of preventing an oil-can effect characteristic ofconventional molding techniques. The oil-can effect is illustrated inFIGS. 9-12. FIG. 9 shows a lower hot press platen 30 and an upper hotpress platen 32 enclosing a mold 34. Following the heating phase, themold is transferred to a cold press. FIG. 10 shows a lower cold pressplaten 36 supporting the mold 34. The lower surface 38 of the flexiblemember 40 of the mold 34 is in contact with the cold lower press platen36 at the same time as the upper surface 42 thereof is in contact withthe hot mold substance 44. This produces a contracting force on thelower surface 38, represented by arrows 46. No such force is developedon the upper surface 42, and the result is a bowing upward of theflexible member 40, as illustrated in FIG. 10. This causes an opening byforce of the mold top 48 and the oozing out or flashing of moltenplastic as indicated at 50.

When the cold press upper platen 52 closes with respect to the lowerplaten 36 under high pressure, much of the flash material 50 is forcedout of the mold assembly, as shown in FIG. 11. During the cooling cycle,the mold cavity volume expands as the bow is removed from the flexiblemember 40 and the upper surface 42 thereof contracts, as shown in FIG.12, leav' ing a space 52.

The resulting conventional molded plaque 54 shown in FIG. 13 isworthless. It has a thin center and poor caliper as well as shrink marks56 and bubbles 58, since it was cooled without being subjectedsimultaneously to pressure. The present invention thus prevents plaqueimperfections resulting from typical single-press techniques and alsothe oil-can effect that characterizes typical transfer moldingtechniques.

Thus there is provided in accordance with the invention novel andhighly-effective molding methods and apparatus facilitating theproduction of a plastic printing plaque that can be either of uniformthickness or somewhat thicker at the center than at the edges and thatis formed without shrink marks, bubbles, or other imperfections. Theprinting plaque can be made of a light-sensitive plastic and need not bebonded to an electrotype shell. The precise control of the maximumlateral distortion of the flexible walls achieved in accordance with theinvention permits a degree of quality control not heretofore attainable.

Many modifications of the representative embodiments of the inventiondisclosed herein will readily occur to those skilled in the art. Forexample, the insulating spaces provided in accordance with the inventionmay throughout most of their area be of greater depth than the maximumpermissible distortion of the flexible walls, and abutment means may beprovided, formed, for example, on the flexible walls or on the pressplatens, for limiting the maximum lateral distortion of the flexiblewalls. Accordingly, the invention is to be construed as including all ofthe embodiments thereof within the scope of the appended claims.

We claim:

1. Molding apparatus for making a flat, generally rectangular plasticprinting plaque, comprising mold means formed with a shallow, generallyrectangular cavity for receiving and molding a plastic substance, saidmold means having a flexible portion and being movable between open andclosed positions, and press means for enclosing said mold means, holdingsaid mold means in said closed position under pressure, and facilitatingmolding of said plastic substance in said mold cavity to form a printingplaque, the relative conformation of said mold means and said pressmeans being such as to leave an insulating space between said flexibleportion and said press means, said insulating space permitting a totallateral deformation of said flexible portion thereinto which is withinthe range of 0.002 inches to 0.020 inches.

2. Molding apparatus according to claim 1 wherein said ran e. is 0.006inches to 0.010 inches.

3. Mo ding apparatus according to claim I wherein said flexible portioncomprises a flexible wall of said mold means and said insulating spacepermits deformation of said wall within said range.

4. Molding apparatus according to claim 1 wherein said flexible portioncomprises two flexible walls of said mold means and said insulatingspace comprises two compartments, said compartments respectivelypermitting deformation of each of said walls within half said range.

5. Molding apparatus according to claim 1 wherein said press meanscomprises a hot press and a cold press for successively enclosing saidmold means, the relative conformation of said mold means on the one handand said hot and cold presses on the other being such as to leave aninsulating space between said flexible portion and said hot press whensaid hot press encloses said mold means and between said flexibleportion and said cold press when said cold press encloses said moldmeans.

6. Molding apparatus for making a flat, generally rectangular plasticprinting plaque, comprising mold means formed with a shallow, generallyrectangular cavity for receiving and molding; a plastic substance, saidmold means having a flexible portion and being movable between open andclosed positions, and press means for enclosing said mold means, holdingsaid mold means in said closed position under pressure, the facilitatingmolding of said plastic substance in said mold cavity to form a printingplaque, the relative conformation of said mold means and said pressmeans being such as to leave an insulating space between said flexibleportion and said press means, said insulating space permitting a totallateral deformation of said flexible portion thereinto which issubstantially equal to 1/10 of 1 percent of one of the major dimensionsof said plaque.

7. Molding apparatus for making a flat, generally rectangular plasticprinting plaque, comprising mold means formed with a shallow, generallyrectangular cavity for receiving and molding a plastic substance, saidmold means having a flexible portion and being movable between open andclosed positions, and press means for enclosing said mold means, holdingsaid mold means in said closed position under pressure, and facilitatingmolding of said plastic substance in said mold cavity to form a printingplaque, the relative conformation of said mold means and said pressmeans being such as to leave an insulating space between said flexibleportion and said press means, said insulating space permitting a totallateral deformation of said flexible portion thereinto which issubstantially equal to 10 percent of the thickness of said plaque.

1. Molding apparatus for making a flat, generally rectangular plasticprinting plaque, comprising mold means formed with a shallow, generallyrectangular cavity for receiving and molding a plastic substance, saidmold means having a flexible portion and being movable between open andclosed positions, and press means for enclosing said mold means, holdingsaid mold means in said closed position under pressure, and facilitatingmolding of said plastic substance in said mold cavity to form a printingplaque, the relative conformation of said mold means and said pressmeans being such as to leave an insulating space between said flexibleportion and said press means, said insulating space permitting a totallateral deformation of said flexible portion thereinto which is withinthe range of 0.002 inches to 0.020 inches.
 2. Molding apparatusaccording to claim 1 wherein said range is 0.006 inches to 0.010 inches.3. Molding apparatus according to claim 1 wherein said flexible portioncomprises a flexible wall of said mold means and said insulating spacepermits deformation of said wall within said range.
 4. Molding apparatusaccording to claim 1 wherein said flexible portion comprises twoflexible walls of said mold means and said insulating space comprisestwo compartments, said compartments respectively permitting deformationof each of said walls within half said range.
 5. Molding apparatusaccording to claim 1 wherein said press means comprises a hot press anda cold press for successively enclosing said mold means, the relativeconformation of said mold means on the one hand and said hot and coldpresses on the other being such as to leave an insulating space betweensaid flexible portion and said hot press when said hot press enclosessaid mold means and between said flexible portion and said cold presswhen said cold press encloses said mold means.
 6. Molding apparatus formaking a flat, generally rectangular plastic printing plaque, comprisingmold means formed with a shallow, generally rectangular cavity forreceiving and molding a plastic substance, said mold means having aflexible portion and being movable between open and closed positions,and press means for enclosing said mold means, holding said mold meansin said closed position under pressure, the facilitating molding of saidplastic substance in said mold cavity to form a printing plaque, therelative conformation of said mold means and said press means being suchas to leave an insulating space between said flexible portion and saidpress means, said insulating space permitting a total lateraldeformation of said flexible portion thereinto which is substantiallyequal to 1/10 of 1 percent of one of the major dimensions of saidplaque.
 7. Molding apparatus for making a flat, generally rectangularplastic printing plaque, comprising mold means formed with a shallow,generally rectangular cavity for receiving and molding a plasticsubstance, said mold means having a flexible portion and being movablebetween open and closed positions, and press means for enclosing saidmold means, holding said mold means in said closed position underpressure, and facilitating molding of said plastic substance in saidmold cavity to form a printing plaque, the relative conformation of saidmold means and said press means being such as to leave an insulatingspace between said flexible portion and said press means, saidinsulating space permitting a total lateral deformation of said flexibleportion thereinto which is substantially equal to 10 percent of thethickness of said plaque.